This invention relates to the field of micromachined pressure sensors, and particularly to micromachined pressure sensors that incorporate diffractive optics as a means of sensing the pressure.
Several varieties of optical pressure sensors are comprised of micromachined sensors or components that serve as prior art to this invention. For example, U.S. Pat No. 4,620,093 describes an optical pressure sensor designed for rocket engine applications. This prior art optical pressure sensor incorporates a micromachined diaphragm etched with a diffraction grating to measure the changes in central order and first order diffracted beams of impinging laser light to monitor the effects of pressure and temperature. Additionally, U.S. Pat. No. 4,932,262 describes a pressure sensor comprising two micromachined elements, one of which is a diaphragm, which sandwich an optical fiber coupled to an optical grating. The variations monitored in the output of the fiber reflect the pressure variations induced on the diaphragm.
U.S. Pat. No. 5,101,664 describes a prior art optical pressure transducer than is comprised of a micromachined flexible silicon diaphragm fixed to a silicon bridge. The bridge is driven at its resonant frequency which is dependent upon the pressure applied to the silicon diaphragm. Measurements of the bridge""s vibrations are made using interferometric techniques. U.S. Pat. No. 5,105,665 also describes prior art based on the vibrating bridge fixed to a diaphragm style of optical sensor. This prior art also uses optical means to interpret the pressure on the diaphragm from the vibration of the bridge. U.S. Pat. No. 5,152,173 also provides prior art of an optically excited and detected micromachined pressure sensor comprised of a vibrating beam mounted on a diaphragm.
The prior art listed above are all optically based pressure sensors which rely on simple intensity reflectance measurements to determine the pressure opposite the deflecting diaphragm. This limits the sensitivity of pressure measurements which can be obtained, in contrast to significantly more sensitive optical pressure measurements that can be obtained by detecting and analyzing optical interference patterns.
U.S. Pat. No. 5,808,210 describes a prior art resonant microbeam pressure sensor that provides frequency based measurements corresponding to strain on the diaphragm supported microbeam. Although the microbeam is integrated into the diaphragm to which external pressure to be measured is applied, the sensing mechanism relies on detecting and accurately measuring the resonant frequency of the microbeam.
In addition to the optically and frequency based sensors described above, there are a variety of micropressure sensors based in semiconductor materials which rely on piezo electric or piezo resistive properties to measure pressure. The limiting factor in piezo based pressure sensors is their inability to be used in applications in which there are electromagnetic fields present or in applications which risk explosion due to their electrical nature. It is in these areas that an improvement to the prior art is clearly needed.
It is therefore desirable to provide an improved optical pressure sensor system which utilizes micromachined diffraction grating components capable of significantly more sensitive pressure measurements.
It is also desirable to provide an improved optical pressure sensor system which utilizes micromachined diffraction grating components capable of application in the presence of electromagnetic fields and explosion risks.
The present invention provides an improved optical pressure sensor over the prior art, by relying on a significantly more sensitive optical interference measurement to determine the desired pressure. The pressure of a fluid in a vessel for which pressure is to be monitored (measured vessel) is related in the present invention to the deflection of a diaphragm in a pressure chamber of the sensor which has an inlet for fluid transfer from the measured vessel. The deflection of the diaphragm is determined by monitoring the interference at critical positions of coherent light reflected from the diaphragm and from a grating structure superimposed over the diaphragm. Intensity detectors are placed at critical positions such as the specific orders of the diffraction grating to measure the interference of the reflected light. The interferometric accuracy with which the pressure measurement is made with the present invention far exceeds that obtained with optical pressure sensors described in the prior art. The interferometric detection is sensitive to motion equivalent to fractions of the wavelength of light used in the system.
The present invention has direct application to sensitive pressure measurements in a variety of industrial situations. The present invention is a viable option as a pressure monitoring system for pipelines and storage tanks, whether remote or local. The digital nature of the measurement results could easily allow for remote monitoring or local storage of results for periodic downloading. In addition, the present invention is an ideal pressure monitor for automated packaging and manufacturing systems such as in the pharmaceutical and chemical industries.